Refractive measurements of the eye should occur with the accommodation of the eye fully relaxed. To accomplish this, aberrometers move an internal visual target to draw the eye to its farthest focus. Then a final refractive measurement is made. When the target is at the optimal position, the target is “fogged” and it always appears slightly fuzzy to the patient. However, sometimes the eye does not respond to the target, and the final refractive measurement can occur with the eye partially accommodated. When this happens, the patient is said to exhibit “instrument myopia” and the target may either appear clear or fuzzy to the patient.
While most people respond reliably to the target inside an aberrometer, some patients persistently exhibit instrument myopia. Repeated measurements can fatigue the eye and the patient can exhibit increasing instrument myopia.
When patients are screened for LASIK treatment, they are measured both with a manifest refraction and with an aberrometer. Typically the manifest refraction is done first, and then the results are entered into the aberrometer software. If the wavefront and manifest refractions agree within some tolerance, the patient may be treated with wavefront guided LASIK. However, if the measurements disagree, the patient can only be treated with standard LASIK, based on the manifest refraction alone.
To ensure the greatest number of patients qualify for wavefront guided LASIK, the aberrometer should minimize instrument myopia, or provide some means to help the doctor to get the patient to relax their accommodation.
Doctors have a number of techniques they can use to coax a patient into relaxing accommodation. For instance, they can distract a patient by telling them to grip a handle, or mentally subtract two numbers. However, when the doctor employs such a technique, a standard measurement follows without any interactive feedback. Consequently the doctor has to wait many seconds to see if the desired effect occurred. If the effect of the coaxing was transitory, the software will still produce a measurement with instrument myopia. Also, coaxing takes time. Prolonged measurement sessions tend to fatigue the eye and often results in the patient showing increasing instrument myopia.